Edit: must hit add reply quicker.
It was explained when you originally asked about all this but we suffered language barrier issues.
Discs are 4. whatever gigabytes (that is how much they hold) and the whole space is written to. When they are dumped all the sectors are copied back.
Games do not have to take up the whole disc so junk data (which the game could care less about) fills the space.
However in the case of the wii this junk data gets altered and this means in nearly all cases it does compress very well at all using conventional methods.
Dack's tool takes this junk and replaces it with data that is simple to compress.